Every component in your system design needs to talk to other components. Clients talk to servers. Servers talk to databases. Services talk to each other.
If you don't understand how this communication works, you'll draw boxes and arrows without knowing what's happening. Your interviewer will ask "How does the client reach your server?" and you'll stumble.
I'll cover DNS, HTTP, API styles, and real-time communication patterns. These appear in every single system design.